The use of a coiled electrode or "jellyroll" cell configuration is well known in the art of galvanic cell construction. Such configuration involves the coiling of two flat electrodes with a layer of separator material disposed therebetween about a common central axis. The coiled electrode assembly is then typically placed in a cylindrical cell container and electrolyte added, thereby producing a cell having an increased area of surface contact between the anode and cathode relative to comparably sized cells having molded cathodes. This increased surface contact will generally permit cells having such a coiled electrode assembly to operate at higher discharge rates than molded-cathode-containing cells.
In general, two approaches have been taken in the past to produce jellyroll cells. The first of these, typified in Japanese Unexamined Patent Application No. 56-160776, involves encapsulating one of the electrodes (typically the cathode) in separator material prior to the winding of the electrode assembly. This encapsulated electrode is then mated with the cell's other electrode and the two wound together into a coiled configuration.
However, this mating step involves a slow, hand operation. This is because the electrode strips are preferably wound as evenly as is practicable in order to produce a coiled assembly wherein the end surfaces have a generally uniform appearance, thereby simplifying insertion and sealing of the coiled electrode assembly into the cell housing. Should such uniform coiling not be achieved, as is likely when an automated mating process is employed, there is an increased risk of the separator tearing during cell assembly and a corresponding high scrap rate will be incurred. These difficulties become more pronounced in cells for which only a minimum amount of variance in height from that of a perfectly coiled assembly is provided within the cell housing. Thus this problem is more associated with smaller cell sizes wherein it is desirable to have as little wasted volume (i.e. volume not occupied by active cell material) as possible. Consequently, this first approach to providing jellyroll cells is not readily adaptable to producing cells, particularly smaller cells, on a large commercial scale. Moreover, when the cathode is the encapsulated electrode and the anode is composed of a fragile material such as lithium, such anode material may flake off its support (e.g. a metal screen) during cell assembly thereby contaminating the cell assembly apparatus as well as reducing the electrochemical output of the cell produced.
The second general approach adopted in the past, typified by the commercial method of manufacture employed by Union Carbide Corporation for the production of nickel-cadmium cells for at least the last decade, involves a process wherein both electrodes are encapsulated in separator material. More specifically, this process involves placing the electrodes on a first strip of separator material in an end-to-end disposition. A second strip of separator material is then placed congruent to the first strip of separator material and the lengthwise edges of the separator strips sealed together. Alternatively the first strip of separator material is doubled in width and folded in half along its length over the electrodes placed in an end-to-end disposition on such first strip. The open lengthwise edge of such folded separator is sealed, thereby encapsulating the electrodes while said electrodes are still in an end-to-end disposition. In either embodiment, the encapsulated electrodes are then superimposed by folding the separator assembly transversely between the ends of the electrodes. This folded assembly is subsequently wound and inserted into the cell housing. However, it has been observed that this method has the disadvantage that if the central transverse fold is not correct, i.e., if such fold varies more than a few degrees from perpendicular relative to the length of the electrode assembly, the two electrodes may not be adequately aligned. Such poor alignment may create difficulty in the winding operation as well as in the insertion and closing of the wound assembly into the cell housing. As is the case with the first method discussed above, these difficulties are more pronounced during the assembly of cells wherein only a minimum allowance for excess height is provided by the cell housing.
Therefore, it is an object of this invention to provide a method for manufacturing a coiled electrode assembly, which method is readily adaptable to high speed automatic manufacturing techniques.
It is a further object of the invention to provide a method for manufacturing coiled electrode assemblies having ends possessing a uniform appearance thereby reducing the amount of scrap incurred in inserting such assemblies into cells.
It is still another object of this invention to provide a coiled electrode assembly for use in galvanic cells which assembly has ends having a uniform appearance such that said cells which employ such assemblies are less likely to be produced with torn separators.
The foregoing and additional objects will become fully apparent from the following description and the accompanying drawings.